The article “A Double-Laser Diode Onboard Sensor for Velocity Measurements”, X. Raoul, T. Bosch, G. Plantier and N. Servagent, IEEE Transactions on Instrumentation and Measurement, Vol. 53, February 2007, discloses a self-mixing interference (SMI) sensor on a car for a real-time velocity measurement. However, this measured velocity can generally not be used in an Electronic Stability Program (ESP) or in another car safety system, because it is not ensured that the measured velocity is reliable and does not trigger a safety relevant action.